Body components or bodies of automobiles are immersed into appropriate baths to undergo treatment or painting. Suitable holes require to be provided to allow the liquid to flow in and to flow out again. Such holes have to be closed afterwards, primarily for protection from corrosion. A large number of different closing plugs have become known to close such openings. The are commonly made of plastics and are mounted by hand by urging them into the opening. To this end, such closing plugs mostly have a flange which leans against one side of the sheet, and locking means which grip behind the other side of the sheet. Since air and/or water is to be prevented from entering the flange sealingly bears against the sheet and an additional sealing agent may be provided, e.g. a hot-melt adhesive. If a hot-melt adhesive material is used it may be contemplated to omit the snap connection and to arrange for the plug to be mounted exclusively by means of the hot-melt adhesive. Finally, plugs are also known which are completely formed from a hot-melt adhesive material. As is known, openings or holes in the area of automobile bodies have a sound transmission characteristic different from that of the material outside the holes. Another sound transmission will also result if a closing plug is employed. Moreover, there is a danger that the closing plug will vibrate, thus producing noise.